A day after parents staged an impromptu rally in Mianzhu on Saturday, the Communist Party’s top local official, Jiang Guohua, came to plead with the protesters to not carry out their plan to march to Chengdu, the provincial capital, where they sought to prevail on higher-level authorities. Mr. Jiang, on his knees, failed to deter the parents, who shouted in his face and continued their march.